Conventionally, the coated cutting tools, in which as a hard coating layer, a Ti—Al-based complex nitride layer is formed on the surface of the cutting tool body made of: tungsten carbide (hereinafter referred as WC)-based cemented carbide; titanium carbonitride (hereinafter referred as TiCN)-based cermet; or cubic boron nitride (hereinafter referred as cBN)-based ultra-high pressure sintered material (hereinafter collectively referred as “body”), by the physical vapor deposition method, are known. These coated cutting tools exhibit an excellent wear resistance.
However, various proposals have been made for improving the hard coating layer since unusual wear such as chipping or the like is prone to occur when coated cutting tools, on which the conventional Ti—Al-based complex nitride layer is formed, are used in high-speed intermittent cutting condition, even though they exhibit relatively excellent wear resistance.
For example, a configuration is proposed in Patent Literature 1 (PTL 1). In the configuration, a hard coating layer, which is made of the Ti and Al complex nitride satisfying 0.35≦x≦0.60 (x is in atomic ratio) in the case where the Ti and Al complex nitride is expressed by the composition formula, (Ti1-xAlx)N, is formed on the surface of the cutting tool body by a physical vapor deposition method. In addition, the hard coating layer has an alternate laminated structure of the thin layer A, which is made of a granular crystal structure of the above-described (Ti, Al)N layer, and the thin layer B, which is made of a columnar crystal structure. In addition to the configuration described above: each of the thin layer A and the thin layer B has layer thickness of 0.05-2 μm; the crystal grain size of the granular crystal is set to 30 nm or less; and the crystal grain size of the columnar crystal is set to 50-500 nm. It is disclosed that the hard coating layer exhibits excellent chipping resistance, fracturing resistance, and peeling resistance in high-speed intermittent cutting work of high hardness steel by satisfying the configuration described above.
However, in this coated cutting tool, it is difficult to obtain the Al content ratio, x, to 0.6 or more since the hard coating layer is formed by a physical vapor deposition method. Thus, further improvement of cutting performance is still needed.
To meet the requirements, a technique, in which the Al content ratio, x, is increased to about 0.9 by forming the hard coating layer by a chemical vapor deposition method, has been proposed.
For example, it is described in Patent Literature 2 (PTL 2) that a (Ti1-xAlx)N layer, the Al content ratio of which is 0.65-0.95, can be formed by performing a chemical vapor deposition in a temperature range of 650-900° C. in a mixed reaction gas of TiCl4, AlCl3, and NH3. What is intended in PTL 2 is improving heat insulating effect by putting an extra coating of the Al2O3 layer on top of the (Ti1-xAlx)N layer. Thus, PTL 2 is silent about any effect of forming the (Ti1-xAlx)N layer with the increased x value to 0.65-0.95 on the cutting performance itself.
In addition, a configuration is proposed to improve heat resistance and fatigue strength of a coated cutting tool in Patent Literature 3 (PTL 3). In the configuration, a TiCN layer and an Al2O3 layer are provided as an inner layer. A (Ti1-xAlx)N layer (x is 0.65-0.9), which is in a cubic crystal structure or a cubic crystal structure including a hexagonal crystal structure, is coated on the inner layer as an outer layer by a chemical vapor deposition method. In addition, compressive stress of 100-1100 MPa is given to the outer layer.